Can a conspirator be held liable for a substantive crime committed pursuant to the conspiracy?

b. A conspirator cannot be held liable for a substantive offense committed pursuant to the conspiracy if the offense was committed before he joined the conspiracy. (People v. Weiss (1958) 50 Cal.2d 535, 564-565, 327 P.2d 527; see generally Perkins, The Act of One Conspirator (1974) 26 Hastings L.J. 337, 344-345.) The trial court, however, did not instruct the jury that it must find defendant joined the conspiracy before the murder. A trial court has a sua sponte duty to instruct on the general principles of law relevant to the issues raised by the evidence. (People v. Wilson (1967) 66 Cal.2d 749, 759, 59 Cal.Rptr. 156, 427 P.2d 820.) There was evidence suggesting that defendant did not join the alleged conspiracy until after the murder.